Eq. 23.13 ). The difference between the calculated PAO2 and the measured PaO2 is the AaDO2. In individuals with normal lungs who are breathing room air, the AaDO2 is less than 15 mm Hg. The mean value rises approximately 3 mm Hg per decade of life after 30 years of age. Hence, an AaDO2 lower than 25 mm Hg is considered the upper limit of normal.
Assume that an individual with pneumonia is receiving 30% supplemental O2 by face mask. Arterial blood gas pH is 7.40, PaCO2 is 44 mm Hg, and PaO2 is 70 mm Hg. What is the patient’s AaDO2? (Assume that the patient is at sea level and the patient’s respiratory quotient is 0.8.) According to the alveolar air equation (
Eq. 23.13
R 44PAO2 = [0 3. × (760 − 47)] − 0 8. = 159 mm Hg
Therefore,
This high AaDO2 suggests that the patient has lung disease (in this case, pneumonia).
Abnormalities in PaO2 can occur with or without an elevation in AaDO2. Hence, the relationship between PaO2 and AaDO2 is useful in determining the cause of an abnormal PaO2 and in predicting the response to therapy (particularly to supplemental O2 administration). Causes of a reduction in PaO2 (arterial hypoxemia) and their effect on AaDO2 are listed in
Table 23.2 . Each of these causes is discussed in greater detail in the following sections.
Arterial Blood Hypoxemia, Hypoxia, and Hypercarbia
Arterial hypoxemia is defined as a PaO2 lower than 80 mm Hg in an adult who is breathing room air at sea level. Hypoxia is defined as insufficient O2 to carry out normal metabolic functions; hypoxia often occurs when the PaO2 is less than 60 mm Hg. There are four major categories of hypoxia. The first, hypoxic hypoxia, is the most common. The six main pulmonary conditions associated with hypoxic hypoxia—anatomical shunt, physiological shunt, decreased FiO2, V̇ /Q̇ mismatching, diffusion abnormalities, and hypoventilation—are described in the following sections and in
Table 23.2 . A second category is anemic hypoxia, which is caused by a decrease in the amount of functioning hemoglobin as a result of too little hemoglobin, abnormal hemoglobin, or interference with the chemical combination of oxygen and hemoglobin (e.g., carbon monoxide poisoning; see the following “
In the Clinic” box). The third category is hypoperfusion hypoxia, which results from low
CHAPTER 23 Ventilation, Perfusion, and Ventilation/Perfusion Relationships
AaDO2, alveolar-arterial difference for oxygen; FiO2, fraction of inspired oxygen; PaO2, partial pressure of arterial oxygen.
blood flow (e.g., decreased cardiac output) and reduced oxygen delivery to the tissues. Histotoxic hypoxia, the fourth category of hypoxia, occurs when the cellular machinery that uses oxygen to produce energy is poisoned, as in cyanide poisoning. In this situation, arterial and venous PO2 are normal or increased because oxygen is not being utilized.